The present invention generally relates to radiotelephone communication systems and more specifically to a method and apparatus for assigning voice channels in cellular radiotelephone systems having colocated base sites.
Mobile radiotelephone service has been in use for some time and traditionally has been characterized by a central site transmitting by way of high-power transmitters to a limited number of mobile or portable radiotelephones in a large geographic area. To avoid repetition, the word "mobile" will be used hereinafter to mean mobile and/or portable radiotelephones. Mobile transmissions, due to their lower power transmitters, were generally received in previous systems by a network of satellite receivers remotely located from the central site for receiving and relaying mobile transmissions to the central site for processing. In previous systems only a limited number of radio channels were available, thus limiting the number of radiotelephone conversations in an entire city to the specific number of available channels.
Modern cellular radiotelephone systems have a comparatively large number of radio channels available which, further, can be effectively multiplied by reuse of the radio channels in a geographical area, such as the metropolitan area of a large city like Chicago or New York, by dividing the radio coverage area into smaller coverage areas called "cells" using low power transmitters and coverage restricted receivers. Such cellular systems are further described in U.S. Pat. Nos. 3,906,166 and 4,268,722. The limited coverage area enables the radio channels used in one cell to be reused in another cell geographically separated according to a predetermined plan, such as a four cell pattern shown and described in U.S. Pat. No. 4,128,740. In this four cell pattern, each cell is assigned a subset of the available radio channels and reuse of the radio channels is accomplished by repeating the pattern throughout a geographical area.
A cellular system typically utilizes a pair of radio frequencies for each radio channel in each cell. Each cell is assigned at least one paging/access channel and several voice channels. The paging/access channel is dedicated to controlling the operation of the mobiles by means of data messages transmitted to and received from the mobiles. Control functions performed include paging selected mobiles, receiving requests for service from mobiles, instructing mobiles to tune to a voice channel where a conversation may take place, identifying the particular system to the mobiles, and enabling mobile registration by which process the mobiles identify themselves to the system. The data message and radio channel specifications for U.S. cellular radiotelephone systems are set forth in Electronic Industries Association (EIA) Interim Standard IS-3 implemented in accordance with 47 CFR 22 and the Report and Orders pertaining to Federal Communications Commission Docket 79-318. Copies of EIA Interim Standard IS-3 may be obtained from the Engineering Department of the Electronic Industries Association at 2001 Eye Street, N.W., Washington, D.C. U.S.A. 20006.
A cellular system can grow by adding more voice channels to existing cells, subdividing existing cells into smaller cells and adding new cells. The least costly and therefor preferable way to grow is to add voice channels to existing cells. However, the number of voice channels that may be added is limited to the maximum number of voice channels which conventional cell site equipment can accommodate. Since some of the additional voice channels may be in new radio frequency bands accessible only to some of the mobiles, the process of assigning a voice channel to a mobile must take into account whether or not the available voice channel can be accessed by that mobile. Accordingly, there is a need for an improved method and apparatus for accommodating and assigning additional voice channels in existing cells of cellular systems without subdividing existing cells or adding new cells.